Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The atomic structure of semiconductor material allows its electrical conductivity to be manipulated by the application of an electric field or base current or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed calculations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing, and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual die from the finished wafer and packaging the die to provide structural support and environmental isolation.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller die size can be achieved by improvements in the front-end process resulting in die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
FIG. 1 illustrates a portion of flipchip type semiconductor device 10 with interconnect 12 metallurgically and electrically connected between bump pad 14 and trace line 20 using solder mask 15. A circular solder mask or registration opening (SRO) 16 is formed over substrate 18 to expose trace line 20, as shown in FIG. 2. Trace line 20 is a straight conductor with optional bump pad for mating to interconnect 12. SRO 16 confines the conductive bump material on the bump pad of trace line 20 during reflow and prevents the molten bump material from leeching onto the trace lines, which can cause electrical shorts to adjacent structures. SRO 16 is made larger than the trace line or bump pad. SRO 16 is typically circular in shape and made as small as possible to reduce the pitch of trace line 20 and increase routing density.
In typical design rules, the minimum escape pitch of trace line 20 is limited by the fact that SRO 16 must be at least as large as the base diameter (D) of interconnect 12 plus a solder mask registration tolerance (SRT). In addition, a minimum ligament (L) of solder mask material is needed between adjacent openings by virtue of the limits of the solder mask application process. More specifically, the minimum escape pitch is defined as P=D+2*SRT+L. In one embodiment, D is 100 micrometers (μm), SRT is 10 μm, and L is 60 μm, hence, the minimum escape pitch is 100+2*10+60=180 μm.
FIGS. 3a and 3b show a top view and cross-sectional view of another conventional arrangement with trace line 30 routed between traces lines 32 and 34 and bumps 36 and 38 on substrate 40. Bumps 36 and 38 electrically connect semiconductor die 42 to substrate 40. Solder mask 44 overlays bump pads 46 and 48. The minimum escape pitch of trace line 30 is defined by P=D/2+SRT+L+W/2, where D is bump base diameter, SRT is solder mask registration tolerance, W is trace line width, and L is the ligament separation between SRO and adjacent structures. In one embodiment, D is 100 μm, SRT is 10 μm, W is 30 μm, and L is 60 μm. The minimum escape pitch of trace lines 30-34 is 100/2+10+60+30/2=135 μm. As the demand for high routing density increases, a smaller escape pitch is needed.